1. Field of the Invention
This application is directed to microwave ovens and, more specifically, to microwave ovens which incorporate means for more evenly distributing microwave energy within the oven enclosure, thereby resulting in more even heating and cooking of food prepared in the oven.
2. Description the Prior Art
One well known problem associated with conventional microwave ovens concerns the uneven distribution of microwave energy within the cooking cavity. The result of such unevenness has been the creation of "hot spots" and "cold spots" at different finite areas of the oven. For many types of foods, cooking results are unsatisfactory under such conditions because some portions of the food may be completely cooked while others are barely warmed.
One explanation for the non-uniform cooking pattern is that electromagnetic standing wave patterns, known as "modes, " are set up within the cooking cavity. When such a standing wave pattern is set up, the intensities of the electric and magnetic fields vary greatly with position. The precise configuration of the standing wave or mode pattern during a cooking cycle is dependent on a multitude of factors among which are the characteristics of the microwave energy source of the dimensions and makeup of the cavity and the loading effect of different types and quantities of food which are placed in the cooking cavity.
In an effort to alleviate the problem of non-uniform energy distribution, a great many approaches have been tried with varying degrees of success. The most common approach involves the use of a so-called "mode stirrer" which typically resembles a fan having metal blades. The mode stirrer rotates and may be placed either within the cooking cavity itself (usually protected by a cover constructed of a material transparent to microwave energy) or, to conserve space within the cooking cavity, may be mounted within a recess formed in one of the cooking cavity walls, normally the top.
The function of the mode stirrer is to continually alter the mode pattern within the cooking cavity. If a particular mode exists for only a short time period, if different hot and cold spots are associated with each mode, then, energized over a period of time, the energy distribution within the cavity is more uniform. Other similar arrangements for the same purpose include rotating blades (U.S. Pat. No. 3,692,967), a rotating plate (U.S. Pat. No. 2,909,635), rotating slotted discs (U.S. Pat. No. 3,746,823) and rotating cylinders (U.S. Pat. No. 3,439,143).
In addition to the non-uniformity caused by the particular electric field mode established within the cavity, an additional non-uniformity results from the effects of the food mass being cooked on the distribution of energy within the cavity. More specifically, food is cooked within a microwave oven cavity by absorption of energy reflected from the walls of the cavity. A relatively small mass of food, if centered within the cavity, will be exposed to substantial reflections from all the walls of the cavity. In particular, such small food masses will not only receive direct and reflected energy on the portions thereof facing the feed means, but also will absorb a substantial amount of energy reflected from cavity walls on the opposite side of the cooking cavity from the feed means. The absorption of energy from walls opposite to the microwave energy feed entrance is particularly important in obtaining uniform cooking.
On the other hand, when large food masses such as a large roast are cooked, only a very small amount of energy reflected from the walls of the cavity opposite the feed passage reaches the food. Since energy is attenuated to a great extent by relatively large absorption on the sides of the large food mass nearest the feed opening, only a little is available for reflection from the opposed walls. The result is a food mass which is cooked in a non-uniform manner, i.e. cooking which is non-uniform throughout the mass of the food.
A similar problem is associated with two-shelf cooking arrangements in microwave ovens, since energy to the food on the lower shelf (assuming top feed means) is substantially reduced because the food on the top shelf intercepts a disproportionate share of the available microwave energy.
One attempt to solve this problem is shown in U.S. Pat. No. 2,909,635 in which the waveguide feeding the cooking cavity is designed to have two branches, a first feeding the upper portion of the cavity and a second feeding the lower portions thereof. Energy splitting means are provided in the waveguide to separate the electromagnetic energy into two unequal portions. This approach requires a substantial redesign of the waveguide structure of the microwave oven and is incompatible with a conventional single opening feed system. U.S. Pat. No. 3,320,396 teaches a similar approach to using a dual branched waveguide, but utilizes an antenna feed instead of an aperture for introducing microwave energy into the cavity.